The immune system of vertebrates has developed an exquisite mechanism to detect and eliminate cells within the body that have become infected by viruses. Viral proteins being produced within the infected cells are broken down into peptides by intracellular proteolytic enzymes. Some of the peptides are enfolded by a particular class (Class I) of proteins of the major histocompatability complex (MHC) of genes and are transported to the cell surface, where the viral peptide/MHC protein complex is displayed as a surface antigen. Circulating cytotoxic T lymphocytes (CTLs) having the appropriate specificity recognize the displayed MHC Class I antigen as foreign and proceed, through activation and a complex lytic cascade, to kill the infected cell.
The MHC Class I proteins are expressed in essentially all nucleated cells of the body and are a key element in the immune system's ability to distinguish between “self” molecules and “foreign” (non-self) molecules. They can be distinguished from the other class of proteins of the major histocompatability complex of genes, known as MHC Class II proteins. In humans, the MHC proteins are also known as HLA (human lymphocyte antigen) proteins; in mice the MHC proteins are also known as H-2 proteins.
The MHC Class II proteins are expressed only on certain of the cells of the immune system, including antigen presenting cells (APCs), some macrophages, follicular dendritic cells and many T and B lymphocytes. Unlike Class I proteins, the MHC Class II proteins become associated with peptides that come from materials outside the cytosolic compartment of the cell. For example, when a macrophage engulfs a bacterium it remains in a vesicle, where the bacterial proteins are broken down into peptides, which peptides bind to Class II proteins to form an antigen complex which migrates to the cell surface, to be exposed to the other components of the immune system.
Although MHC Class I antigens are a magnificent mechanism for combating infection, they also are primarily responsible for the failure of tissues, e.g., cells, organs, or parts of organs, that are transplanted from one mammal (donor) to another (host). This rejection of tissue by the host organism was first observed in mouse skin graft experiments in the 1950s and was named the transplantation reaction. The search for the factor on donor cells that was evidently recognized and attacked by the host's immune system led finally to the characterization of the two classes of MHC proteins. See, Snell, G. D., Ann. Rev. Microbiol., 2:439-57 (1957).
Recognition of donor MHC Class I antigens as foreign (non-self) by host CTLs occurs not only where the donor tissue is from a different species (a xenogeneic transplant) but also where the tissues are from a donor of the same species as the host (an allogeneic transplant). The specificity of the T cell receptors on CTLs and other T cells that bind to MHC Class I and Class II antigens is such that a single amino acid difference in the structure of a MHC antigen can be detected as foreign, leading to an immune response. The MHC proteins are expressed from (a) distinct DNA segments (i.e., multiple Class I and Class II genes) and (b) highly polymorphic gene segments with great diversity in the intrinsic coding sequences, which leads to a high degree of polymorphism in MHC proteins. Thus, between genetically unrelated individuals the incidence of MHC proteins matching is only about 1 in 40,000, and the transplantation reaction is only avoided in the case of isogeneic grafts, i.e., the transplantation of tissues between idividuals having a high degree of genetic identity, such as between identical twins or from a parent to first generation offspring. See, Roitt et al., Immunology (2nd ed. 1989), Chapt. 24, pp. 24.1-24.10.
Several methods have been devised to try and overcome the mechanism of MHC Class I antigen recognition and its consequences for the transplanted tissue. Immunosuppressive drugs such as cyclosporin A are employed to block the activation of T cells after binding between the T cell receptor and the MHC antigen has taken place. Another approach to avoiding transplant rejection, known as perfusion, seeks to decoy the components of the immune system that would react with the donor tissue. This approach may be particularly useful for addressing antibodies capable of reacting with foreign-appearing donor tissue. Prior to transplant, tissue from the donor is introduced into the host; the host's immune system recognizes the donor tissue as foreign and T cell proliferation and antibody production ensue; the decoy donor tissue is destroyed, but the host's immune system is thereby partially depleted of cells and proteins, especially antibodies, that are capable of reacting with donor tissue; thereafter, the transplant tissue from the donor is introduced, at a point where the host's ability to react against the donor tissue is reduced. See, e.g., Watkins et al., Transplantation Proceedings, 23(1):360-4 (1991). Another approach to inhibit T cell recognition of donor tissues is to mask the MHC Class I antigens, or block the binding interaction between the antigens and T cells, for example with monoclonal antibodies against the MHC Class I antigens or with soluble ligands of the T cell receptors of that subpopulation T cells that are capable of recognizing the antigens presented on the donor tissue. See, e.g., U.S. Pat. No. 5,283,058 (Feb. 1, 1994). A variation of this approach is to prepare donor tissues in transgenic animals that have been genetically altered to have decreased or eliminated MHC Class I expression. See, Li et al., Transplantation, 55:940-6 (1993); Coffman et al., J. Immunol., 151:425-35 (1993).
Each of these methods can be effective in overcoming rejection or prolonging the survival of donor tissues, but they also have potential drawbacks. Immunosuppressant drugs can lead to serious side effects such as renal failure and hypertension, and they leave the host open to infection and tumor growth that is ordinarily checked by an operating immune system. Use of perfusion, MHC antigen masking, and transgenic donor animals leaves the uninvolved segments of the host's immune system in place, but these methods can be labor intensive (e.g., in the systematic and selective elimination of host antibodies or T cells, in the preparation of specific, individualized antibodies for masking, or in husbandry of transgenic animal donors); and in addition these methods involve tailoring the preparation of donor tissues to overcome the capabilities of the host's individual immune system, that is, they involve the preparation of materials which in general are species and host restricted and are not interchangeable between different hosts.
The present invention seeks to provide a simpler and more flexible alternative to inhibiting the rejection of transplanted tissues mediated by recognition of donor antigens.